The <risk set> at event time $a_j$ contains individuals still under observation and event-free immediately before $a_j$. For group $k$ its size is
$$
r_j^{(k)}=\sum_{i=1}^n\mathbf1\{g_i=k,,x_i\geq a_j\}.
$$
The use of $\geq$ keeps the individual who experiences the event at $a_j$ in the risk set just before that event.

Solution

Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 207 / 4 / a / i / Solution

 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2024/iii/paper-207/4/a/i/solution